1. Field of the Invention
The present invention concerns the field of electronic circuits and, more particularly, means for reactivating a system operating in "sleep" mode.
2. Description of the Background Art
In the present invention, "electronic circuit" means a system including a central processing unit to which at least one peripheral unit may be connected, a system clock for providing a clock signal to all the logic means of the system, in particular to the central processing unit. All the components of the system are supplied by an electric supply source.
Generally, such a system may operate in one of the three following modes: "run" or "active" mode, "stand-by" or "halt" mode, and "sleep" mode.
When the system is in "run" mode, all its components operate. When it is in "stand-by" mode, only the peripheral units operate, and typically the central processing unit is halted, i.e. is no longer activated by the clock signal. And when the system is in "sleep" mode, all the components are halted, including the system clock, and only the supply source keeps the system supplied. Thus, most of the time, such a system operates in "sleep" mode, which allows a significant decrease in its electric power consumption to be achieved.
A problem encountered by the designers of a system of the type described above lies in achieving its reactivation, i.e. the passing thereof from "sleep" mode into "run" mode or "stand-by" mode.
A conventional solution to such a problem consists in providing, via means external to the system, a reset signal for reactivating the entire system. For example, such external means may be formed by a key having means for providing the reset signal to the system, when the key is moved close to the system, and activated by the action of a user.
A drawback of such a solution lies in the fact that it is necessary to use additional means for reactivating the system, which goes against usual industrial criteria as regards cost and space requirements.
Another drawback of this solution lies in the fact that it is necessary to use means external to the system for reactivating it, so that it can not be reactivated autonomously.
Another conventional solution to the aforementioned problem consists in creating a different time base to that provided by the system clock, the latter being deactivated in "sleep" mode. Said time base must be able to measure a time interval as soon as the system passes into "sleep" mode.
By way of example, European Patent Document EP No 586 256 discloses, in the field of mobile telephones, a system including a first clock or system clock operating at a high frequency, and a second clock operating at a lower frequency. The system clock may go into "sleep" mode, i.e. be deactivated for predetermined periods of time. The sleep duration may be measured by counting the number of low frequency pulses provided by the second clock.
A drawback of such a solution lies in the fact that it is necessary to calibrate the low frequency clock with respect to the system clock.
The applicant of the present invention has observed that the conventional solutions such as those described hereinbefore require means dedicated to the operation of the system in "sleep" mode, and that these means require a significant consumption of electric power. Typically such an operation requires a current which is commonly equal to several hundredths of nano-amperes (1 nA=10.sup.-9 A) to several micro-amperes (1 .mu.A=10.sup.-6 A).
An object of the present invention is to provide a system including reactivating means, when the system is in "sleep" mode, this system overcoming the aforementioned drawbacks.
Another object of the present invention is to provide such a system having a low electric power consumption, during its "sleep" mode.
Another object of the present invention is to provide such a system meeting conventional industrial criteria as regards cost and complexity.